The present invention relates to a method of controlling the rotational speed (hereafter referred to as the speed) of an induction motor, and especially to a vector control method without a speed sensor, capable of achieving a highly accurate speed control without a speed sensor, which can obtain high torque from a zero speed range.
In the vector control of an induction motor, the output frequency of a power-conversion unit for the induction motor is usually corresponding to the sum of the speed and the calculated slip frequency. On the other hand, in the vector control method without a speed sensor, the output frequency of the power-conversion unit is controlled with an estimated value of the speed in place of a detected value of the speed. However, since the estimated value of the speed includes an error, the actual slip frequency shifts from the target reference value. In this situation, the magnetic flux (hereafter referred to as the flux) in the induction motor varies according to the torque, and accordingly the torque generated by the induction motor is not proportional to the torque current, which in turn causes a shortage of torque in an extreme case.
Setting-errors in characteristic parameters of the induction motor, which are used for estimating the speed, and changes in the flux in the induction motor, which are caused by the errors, etc., are considered to be the causes of the errors in the estimated speed. A means to effectively correct those changes of the flux has not been devised, and the shortage of torque sometimes happens in the range near zero speed. A report xe2x80x9cSimplified Vector Control System without Speed and Voltage Sensorsxe2x80x94Effects of Setting Errors in Control Parameters and their Compensation -xe2x80x9d by T.Okuyama et al., T. IEE Japan, Vol. 110-D, No. 5, ""90, discloses the effects of the setting errors in the parameters and a means to compensate the effects due to the setting error of the parameters of the induction motor.
An object of the present invention is to provide a method of accurately and efficiently controlling an induction motor without receiving effects of errors in an estimated speed due to changes in constants of the induction motor.
To achieve the above object, the present invention provides a speed-control apparatus for an induction motor for controlling the current output from a power-conversion unit so as to be larger than a current value in the ordinary no-load operation, or the frequency output from the power-conversion unit by calculating a frequency instruction value based on a speed instruction value in place of an estimated speed value if the speed instruction value is less than a predetermined value. By the above control, it is possible to prevent a shortage of torque in the low speed range near zero speed.